Furthermore, the invention relates to a shell, in particular a fuselage shell, a wing shell, a horizontal stabilizer shell or a vertical stabilizer shell, manufactured in accordance with the inventive method, comprising a plurality of stiffening elements applied to a shell skin, for forming component parts for aircraft with high dimensional stability, wherein the stiffening elements and the shell skin are made with epoxy resin from at least partially cured carbon fiber reinforced semi-finished parts.
Due to their low weight and their high mechanical strength, carbon fiber reinforced epoxy resins are increasingly used to manufacture shells for load-bearing component parts of aircraft, such as fuselage cells, airfoil wings, horizontal stabilizers and vertical stabilizers. Such component parts of an aircraft are usually made of at least two shells, which are joined together. For example, a vertical stabilizer is made of mirror-symmetric (half-)shells that are joined together at their respective longitudinal edges.
The shells can be fabricated by several different methods, using so-called “prepregs”. The term “prepreg” means a semi-finished part, made of a curable and carbon fiber reinforced epoxy resin. Such prepregs are ordinarily plate-shaped or strip-shaped, but they may also be sections or profiles with an L-shaped cross section, for example.
Prior to the curing, the prepregs are flexible, and therefore can be brought into virtually any desired shape. Only after curing is completed, the shells for forming component parts for aircraft, that are fabricated with these prepregs, attain their characteristic mechanical properties, such as their extremely high mechanical strength in combination with a very low weight. The curing of the shells made of these prepregs for forming the component parts is ordinarily performed in autoclaves at a temperature of between 120° C. and 180° C. and at pressures of up to 10 bar. The shelf-time of prepregs is limited due to continuously occurring cross-linking processes, and is ordinarily only between 10 and 30 days at room temperature. In the remainder of this description, the term “semi-finished parts” is used instead of the term “prepregs”.
The shells are mainly formed of a shell skin and a plurality of stiffening elements and connecting angle pieces for reinforcement arranged thereon. Typically, two connecting angle pieces extending over the entire length of one stiffening element are arranged for the connection with the shell skin on both sides of each stiffening element. The shell skin, the connecting angle pieces as well as the stiffening elements are all made of semi-finished parts.
A number of manufacturing processes for manufacturing such shells from semi-finished parts are known from the state of the art:
In the so-called “co-curing” process, the entire shell, which includes in particular the shell skin, the stiffening elements and the connecting angle pieces, is enclosed by a single vacuum sack. This vacuum sack is then placed in an autoclave, thereby performing the curing of the entire shell. Thus, in the “co-curing” process, the curing process occurs in a single process step.
The main disadvantage of this process is that it requires a very elaborate apparatus for positioning and fixing the stiffening elements relative to the shell skin during the curing process, which leads to disproportionally high costs, in particular in the case of shells of large dimensions for forming complex component parts, such as airfoil wings, for example.
In the so-called “co-bonding” process, firstly, the stiffening elements are cured in a first step. Subsequently, the not-yet-cured shell skin is placed on the stiffening elements. During the final curing process of the shell skin, the stiffening elements are simultaneously joined with the shell skin. Also the reverse approach, namely first curing the shell skin and then curing the not-yet-cured stiffening elements together with the shell skin, thereby joining them, is also possible in a variation of the “co-bonding” process. In any case, the curing is performed in vacuum sacks placed in an autoclave. In accordance with these two processing methods, the curing process of the shell is performed in two steps.
The main disadvantage of this process variant is that the respectively not-yet-cured component, that is, the stiffening elements or the shell skin, for example, is subject to quality fluctuations, that cannot be ignored. These quality fluctuations may lead in particular to a disadvantageous fiber distribution in the resin matrix, to warping, as well as to pores and trapped air, which is caused primarily by setting movements of the cured component during the curing process.
In accordance with the so-called “secondary bonding” technique, the shell skin as well as the stiffening elements are cured in advance in two separate steps and then joined together by gluing.
The disadvantages of this approach include in particular the higher number of necessary processing steps as well as the necessary fitting precision of the stiffening elements, the connecting angle piece and the shell skin, which ordinarily cannot be ensured with shells of larger dimensions.